Low Energy Transmission Grating with the ACIS-S detector -- Carbon
K edge region
The problem was first noticed in a GO observation (see Figure
1). This feature was subsequently detected in the calibration observation
of 3C 273 (obs ID 1198). The residuals of the GO target and 3C 273
are compared in Figure 1 The similarity is quite striking and indicates
that the difference was intrinsic to the instrument. [The drop beyond
55 A is due to the event threshold cutting into the PH distribution, so
it is not likely to be intrinsic to the source.] The LETG/HRC-S data
from 3C 273 taken immediately prior to the LETG/ACIS-S observation do not
show a comparable residual, indicating that the feature is not entirely
intrinsic to the source. See the LETG/HRC
spectrum from Jeremy Drake and Deron Pease showing a 2PL model; the
(+1 order) residuals do not show a jump near 43 A. The -1 side is
Figure 1. Comparison of two LETG/ACIS-S
observations of AGN. The residuals give the fractional differences
between the simple power law spectral model that was folded through the
instrument response and the observed count spectra. Note the apparent
edge at about 43 A, which is near the K edge of carbon. The gap near
36 A is due to a gap between the S1 and S2 CCDs. The upper target
is a GO target and the lower one is 3C 273 (obs ID 1198).
The data in Figure 1 were analyzed
using released, pre-flight calibration products. More details of
the 3C 273 fits are shown in two PS files giving the 0-30
A region and the 30-60
A regions separately for both the plus side and the minus sides.
The spectral fit was a simple power law with a photon index of 1.6 and
Galactic absorption of 1.7e20 cm2. For this column density, the edge
at C-K would only be about 10%, substantially greater than the observed
edge. The model of the C-K edge is clearly inadequate in these released
calibration files, so calibration files wIth higher spectral resolution
were obtained from the ACIS team. Figure 2 shows
the new model of the S3 QE in the C-K region, improved primarily due to
the improved resolution of the optical blocking filter (OBF) model.
Figure 2. New and old ACIS BI quantum
efficiency models compared. The main difference is in the improved
resolution of the OBF model. Note the resonance absorption features
at 0.2851 and 0.2866 keV and the EXAFS at E > 0.288 keV. The new
OBF model was supplied by George Chartas at PSU and the ACIS QE models
were supplied by Catherine Grant and Mark Bautz of MIT.
In early May, data from a much brighter source, XTE J1118+48,
were obtained. The data will go public in mid-August and will be
available for all users. Because it's so bright at 0.28 keV, the
data were not binned, so that the filter model could be examined in detail.
3 shows the C-K region and how the location of the resonance absorption
features don't quite match the model. A shift of 0.5 eV was required
to get a decent fit. In addition to the energy shift, the depth of
the resonance features was tested by extrapolating a simple model from
lower energies. The result is that a 13% increase in the optical
depth of the resonance features gives an agreement shown in Figure 4.
The filter was decomposed
empirically in order to determine the transmissions of each filter
component so that the C-K edge could be adjusted independently of the other
components in the model.
Figure 4 (portion). Click on this fragment to get the full version.
The absorption above the edge was fit to a simple model
where the opacity decreases as 1/E^9, which is somewhat steeper than expected
if due only to ionization of neutral material. Figure
5 shows the fit of the data to this model and Figure
6 shows that the FI chip shows the same overall level of absorption
in the .29-.33 keV range. Thus, the new absorption component is not
limited to the BI chip and if it is due to the OBF or contamination on
the OBF, then the new component is rather uniform.
Although the origin of the new absorption component is
not known, it appears that it is not associated with a particular CCD type,
which don't have carbon in them anyway, so I have constructed a new OBF
model based on the original
from George Chartas that includes this new absorption component
as well as fixing the energy shift and adjusts the optical depth by 13%,
as described earlier. The IDL
code is available as well as the new
filter model, which I have pruned to an accuracy of 0.1% using Dan
pruning routine. It is compared to the version released by George
Chartas in Figure
Caveats and Conclusions
There are a number of possible problems with this analysis
that I have tried to address and a few other points bear discussion.
The 0.0005 keV shift that is needed to line up the observed
and modelled resonance features may be partially explained by systematic
errors in the wavelength scale of the data, rather than the model.
The sense of the correction is that the OBF energies near C-K are reduced
-- equivalently, one could increase the energies of the events or decrease
their wavelengths. Although a final conclusion hasn't yet been reached,
there is evidence that the LETG/ACIS-S wavelengths are systematically too
large by about 0.1(1)%, which would be an error of 0.28eV -- about 50%
of the required energy shift. The cause of the wavelength error has
not yet been established firmly.
Pulse height (PH) extraction was quite liberal but should
not have a sudden change across the C-K edge, so it is unlikely to cause
the apparent absorption. The pre-flight gains and offsets were applied
to the PH data and then the event energies were corrected on a node-by-node
basis to provide better agreement with the energy inferred from the dispersion
by the LETG. The PH selections should be photometric at a level of
Bad columns are flagged using a 50-pixel running median filter.
There are very few, so the data were eliminated but the exposure function
wasn't adjusted to account for them.
If the new absorption feature were due to carbon in a material
similar to polyimide, then the additional optical depth, 1.34, can be compared
to the optical depth due to the carbon (as decomposed)
at 0.2875 keV in the OBF: 1.1. Taking the fraction of C by mass in
polyimide, 0.72, there is an equivalent thickness of 1440 A of carbon in
the filter; thus, the new absorption component would require about 1750
A of carbon in a compound such as polyimide, or 2435 A of polyimide.
This is substantially greater than the estimated contamination due to nonvolitile
residues estimated by Lorraine Ryan -- 125-250 A -- in an e-mail
Steve O'Dell on 4/27/00.
Intrinsic absorption is not well known but was estimated
to be of order 1.0e20 per cm^2, so the jump at the C-K edge that is intrinsic
to the source should be of order 4%, based on a cosmic abundance of carbon
(3.6e-4) relative to hydrogen. It is not yet clear whether there
are associated resonance features, because some of the carbon along the
line of sight would be in grains and even atomic carbon might well have
some resonant absorption features. Current experts (such as Brendan
McLaughlin) do expect some resonant edge structure, so I may have included
some part of the ISM term into the 13% filter adjustment, which is the
equivalent of 187A of carbon alone. I estimate that ISM with NH of
1e20 would have an equivalent thickness of ~50 A, so ~30% of the adjustment
may be due to the ISM. Better theoretical models of the resonant
absorption at C-K may help. The ISM cannot explain the excess absorption
above 0.2867 keV, however, because it would require a x30 overabundance
of carbon relative to hydrogen and because of the LETG/HRC-S result.
Background was not subtracted from any of the spectra because
the source (XTE J1118) was very bright and the background was, in general,
very small. The background will be important only in the .29 keV
region in the +1 spectrum which is detected on a FI detector. This
could explain why the FI seems to deviate from the BI in
6 except that the agreement above 0.31 keV would then be in question.
Even if the 13% thickness adjustment needed for agreement
in the resonance absorption features is associated with the excess absorption
at energies above 0.2867 keV, then the material has a very different C-K
absorption profile than the carbon in polyimide. The data are consistent
with the possibility that there is only continuum absorption and no resonant
lines, which seems peculiar because even pure carbon should show the resonance
features. I have heard no plausible explanations for how this can
I have compared the flight ACIS and HRC filter models using a decomposition
of the +1, thin Al HRC filter model provided on the
UV ion shield calibration page. I also compared the decompositions
with predictions from the relatively new transmission model for the HETGS
polyimide provided by Kathy Flanagan (optical-constants.990510.tbl). The
results are in one PS
file. I assumed that the polyimide thicknesses were 2000 A for the
OBF and 2050 A for the HRC filter. For Al thicknesses, I used 1300 A for
the OBF and 270 A for the HRC. The HETGS model is normalized to 10 A, according
to the header documentation. Briefly, for each of 6 energy bands, there
are some discrepancies.
In summary, one can get a good agreement in the expected Al opacities if
the thickness of the Al in the HRC UV ion shield is increased about 10%
to 300 A, which is within the Luxel measurement error. The overall shapes
of the edges agree well and the energies of the edges agree to 0.2 eV at
Al-L, C-K and N-K. There are two cases where an energy shift is required
to bring filter models into agreement. There is no simple scaling that
brings the edge opacities due to the polyimide components into complete
agreement; values between 0.75 (C-K and O-K resonance features) and 1.1
(N-K) were obtained. There seem to be kinks in the ACIS opacity models
that appear at energies where the grid spacing changes that may be due
to measurement setup calibrations. There are some other odd differences
that have no obvious explanation.
Below the Al-L edge near 0.073 keV, all filter opacities are dominated
by C and O absorption which I have modelled using fitting constants in
Astrophysical Quantities (4e, p. 109) and the fractional areal densities
of C and O expected using the polyimide formula given in the ACIS calibration
report v2.20, p. 273: C22 H10 O4 N2. Results are shown in the top two panels
of the first figure. The prediction from the HETGS and the OBF agree but
are a x1.42 below the opacity observed in the 0.06-0.07 keV region. The
prediction of the HRC opacity below the Al-L edge requires a 10% increase.
In the lower two panels of the first figure, the predicted opacity due
to polyimide is subtracted from the modelled filter opacities in order
to estimate that component due to Al-L opacity alone for both the HRC and
ACIS filters. The result is that the energies of the features agree well
but that the magnitude of the opacities differ by ~10% in the 0.073-0.30
keV region. No single correction brings these into agreement at the 1%
The C-K region is compared in the second figure in the PS file. The HRC
and ACIS filter models show good agreement in the energies of the resonance
features between 0.285 and 0.290 keV and in the overall shape in the 0.29-0.36
keV region. The magnitude of the opacity differs 25%, however in the resonance
features but only 3% in the .305-0.360 keV region. The ACIS filter model
shows features in the 0.29-0.30 keV region that the HRC filter does not
The N K region is compared in the third figure in the PS file. The two
filters agree very well on the shape of the N opacity except for two places:
0.395-0.400 keV, where the HRC model shows opacity while the ACIS and HETGS
polyimide models do not, and at 0.45 keV where there is a 1% "kink" in
the ACIS model that is not observed in the HETGS and HRC polyimide models.
The kink occurs at a point where the sampling frequency in the ACIS model
changes from 0.2 eV to 0.5 eV, so it appears that two different measurement
runs were used. Overall, a 10% increase in the ACIS/HETGS prediction is
required to match the HRC model.
The O K region is compared in the fourth figure. The HRC and HETGS models
agree on the energies of the resonance feature at 0.531 keV while the ACIS
edge has to be shifted by 0.8 eV to match the HRC and HETGS. The overall
structure of the opacity is quite similar in all models except for a kink
in the HETGS model at about 0.537 keV. If one normalizes the opacity of
the resonance features, the HETGS model has to be shifted by x1.3 to match
the HRC opacity model but the shift is x0.8 if normalized in the 0.54-0.64
keV region. The HRC and ACIS opacities agree better but require a scaling
of 0.75-0.88. Again, there is a 4.5% kink in the ACIS model -- at 0.56
keV in this case -- which can be related to a change in the sampling size
of the ACIS model.
The Al K region is compared in the fifth figure. There is an overall energy
shift of 8.0 eV required to match the shapes in the 1.575-1.66 keV region.
A scaling of 1.13 is also required. The ACIS filter shows a feature that
is not present in the HRC model at 1.570 keV.
I am currently in the middle of checking this new filter
model against other LETG/ACIS-S data, including 3C 273, a GTO observation
of PKS 2155-304 and the GO observation where this effect was originally
found. Preliminary indications are that the adjusted model works
well for PKS 2155 but that it slightly overcorrects the data of the GO
target and 3C 273. I am also working on an adjustment to the BI QE
model because there is a definite discrepancy of 5-20% between the FI and
BI data in the 0.6-1.3 keV band. This adjustment has been worked
out for the XTE J1118 data and seems to work for 3C 273 and PKS 2155-304.
Update -- Feb. 26, 2001
I have determined an empirical correction to the ACIS-S QE model
that accounts just for the anomalous C K edge. Using two observations
of the BL Lac object PKS 2155-304, I have the following
Figure 8 shows the spectra obtained
when using the pre-flight releases of the detector QE. The absorption
edge at C K is clear and present in both observations. The dotted
lines are simple power law fits to all the coarsely binned data.
Figure 9 shows a set of simple fits to these
data. The top panel gives the ratio of the two spectra to the individually
fit power law spectra. The residuals are consistent, so the data
were combined using an uncertainty weighting. The middle panel (shown
below) gives the result of combining the observations and shows a simple edge
model fit that requires accounting for a systematic error in the spectral
slope. The bottom panel shows the residuals from this fit, which
show some systematic deviations that are partly related to the detector
The strength and shape of the edge are consistent with the
results obtained from the observation of XTE J1118+480. The
optical depth at the edge is 1.34, the energy at which the edge appears
is about 0.286 keV, and the spectrum recovers as though the opacity decreases
The edge strength has not changed in the six months between
the two observations (May 2000 to December 2000).
The edge does not appear to have associated anomalous O K
The two observations took place at slightly different off-axis
angles (+1.5' vs -1.5') and the results are consistent.
of Fig. 9, which shows the 0.2-0.4 keV region
and the model for the anomalous C-K absorption edge.
The model fit is available as an ascii
file that can be used as an auxiliary filter for LETG/ACIS-S QE modelling. The functional form is very smooth except at the edge, so it has
been "pruned" using Dan Dewey's IDL software and should still be
accurate to 0.1% (which is smaller than the systematic residuals).
We do not yet know if this auxiliary QE model should be
used for observations
near the center of the ACIS-S array, because all
are offset close to the readout rows.
Contact Herman L. Marshall
(email@example.com) for further information.
Last modified: 8/4/00